Lower main idle air bleeds

[fairwayhit]

I am working through the book (recipe #2) on a 17058272

I have a question about the Main Idle bleeds.  The book says to make both the upper and lower bleeds .070.  I measured the uppers at .073.  Should I make the lowers smaller (like .067) to average .070?

Right now the lower ones have brass restrictors in them and are sized at .040.
I have been reading online that the main idle bleeds have a big impact on the whole fuel curve.  I live at 6500 feet above sea level.  Do I need to leave the lower air bleeds a little smaller? 

[fairwayhit]

I wrote main "idle" air bleeds, but I meant "main air bleeds"  - 

[firefox3]

What size engine? Which rods and jets did it originally come with?

[Ethan1]

What size engine? Which rods and jets did it originally come with?

 Engine for that carb was a 301cid Pontiac and stock specs of that carb were, .072" Primary Metering Jets, 51P Primary Metering Rods, "K" Secondary Hanger and CH Secondary Metering Rods.

[fairwayhit]

It is going on a Pontiac 400.  Cliff sent me some 71 jets and a set of his rods.

I don't understand why there are two sets of main bleeds.   The lower bleeds and Upper bleeds look like they do the same thing.  If the upper bleeds are a little big, should the lowers be smaller to average it out?

[firefox3]

First off, I'm not trying to fan any flames here, but I would never automatically just drill out my air bleeds to .070" because some book told me to do so.

Also, air bleed sizes aren't about "averaging".

I've seen a lot of Quadrajets that had smaller bleeds in the main body than what was in the air horn. Rochester did this to fine tune the fuel curve for a specific engine/vehicle combo.
Reducing the .120" bleeds, found in a lot of the 70's carbs., to .070" is a good starting point for a mild 350 Chevrolet engine as Cliff points out in his book.

I would be very surprised if your 71 jet/44 rod combo wasn't too lean for your 400 using .070" upper and lower bleeds.

[fairwayhit]

I forgot to mention.  I am at 6500 feet above sea level.

The motor is going to be a mild pontiac 400 with 9:1 compression and a 200r4 transmission.  Nothing crazy.

How critical is the main bleed size anyway? Any suggestion for what size to make the main body bleeds?

[firefox3]

It should be calibrated leaner, especially for that far above sea level. Holley carbs. are typically jetted down one size for every 2,000 ft. gain in altitude.

I think the rods you got from Cliff are supposed to be tapered about .010", though I've never used them before. That gives you quite a bit of adjustment for your cruise air/fuel ratio.

Personally, I would try the 71 or 72 jet/44 rod combo with the .073" upper & .040" lower bleeds.
With the secondaries locked out(or disconnected), do some full throttle testing in order to establish what size jet your engine seems to run the best with. 
Then try to get your cruise air/fuel ratio to your liking by adjusting the APT screw. This changes the portion of the rod that will be in the jet. CW for leaner cruise...CCW for richer cruise.
If it's still too rich at cruise, I would increase the size of the lower bleed by .002" at a time. Of course, doing this will also alter your WOT fuel/air ratio.

Another thing that'll probably help is to install a stronger power piston spring as the ones in most late 70's carbs. are usually pretty weak.

[firefox3]

My bad...Cliff's rods are .043" at their widest point, not .044".

Still I would try what I suggested above.

In addition, since you already have 51P rods, I would try them last before enlarging the lower bleeds just to see if they get you in the ballpark at cruise or if the engine is just too lean with them. If you end up running a jet larger than 72, they may work with some APT adjustment.

[Cliff Ruggles]

I'd leave the MAB's at the stock sizes and use the 71 jets and tapered metering rods that we sent.  It will be close at your altitude for what you are doing......Cliff